The source of color in pulp and paper wastewater is primarily lignin. Lignin is a sticky substance that binds the cell structures of plants together giving them strength, rigidity and hardness. Without lignin, trees and other vegetation would not have the necessary rigidity to stand erect. Lignin becomes soluble when wood pulp is processed with strong caustic and acidic chemical solutions. The lignin may be separated and removed in several washing processes.
Lignin is very difficult to biodegrade. It is a complex polymer consisting of phenylpropane units. The phenylpropane units are highly cross-linked, perhaps as many as 500 times. This complexity inhibits microbial degradation for most organisms except primarily white rot fungus.
White rot fungi are a classification of microorganisms that are able to degrade lignin. Their ability to degrade lignin is a result of their secretion of highly oxidative ligninonlytic enzymes consisting primarily of phenoloxidases. The phenoloxidase enzymes believed to be of importance in this process are manganese peroxidase, lignin peroxidase, manganese-independent-peroxidase and laccase. These enzymes break the phenylpropane links of lignin creating much smaller compounds. The smaller compounds are called degradation by-products and are easier for bacteria to use as a food source to further degrade into carbon dioxide and water. The peroxidase enzymes may also assist in degrading cellulose and hemi-cellulose, which are structurally more complex than cellulose. It is known that white rot fungi preferentially produce phenoloxideses over the other enzymes the fungi normally produce when in the presence of glucose.
Normally aerobic measures are undertaken to reduce colorants and biochemical oxygen demand (BOD) in wastewater. Aerobic technologies include trickling filter, activated sludge, rotating biological contactors, oxidation ditch, sequencing batch reactor and even controlled wetlands. By introducing air into the wastewater stream, pumps and other mechanical systems are required which consume significant amounts of electricity. The use of electricity obviously increases the costs of treating wastewater.
Accordingly, an anaerobic, or anaerobic-friendly type of technology is believed to be somewhat advantageous to reduce the costs associated with treating the wastewater. Anaerobic technologies currently available are high-rate systems including continuous-flow stirred tank reactors, contact reactors, upflow sludge blankets, anaerobic filters (upflow and downflow), expanded or fluidized bed and two-stage systems that separate the acid-forming and the methane-forming phases of the anaerobic process. In many prior art systems, aerobic and anaerobic processes are combined into a treatment system. Anaerobic treatment may be used for removing organic matter in high concentration streams, and aerobic treatment may be used on lower concentration streams or as a polishing step to further remove residual organic matter and nutrients from wastewater.
There are numerous problems and difficulties with the prior art systems. First, start up and restart problems can occur with the prior art processes. Secondly, biomass separation, sludge settlement, foaming and scum formation can occur. Odors may be associated with the treatment, excessive biomass may be generated, nitrogen and phosphorous may need to be removed, and any number of other problems have been found with the various prior art systems.
Accordingly, a need exists to provide a more effective color removing method for use with wastewater streams.
Another need exists for an anaerobic process for removing colorants and/or otherwise treating wastewater.